The field of the invention relates power consumption of power amplifier circuits, and more specifically in the area of adjusting gain and current consumption of a power amplifier while maintaining amplified signal linearity.
Power amplifiers are used for amplifying low intensity, low amplitude electrical signal in order to produce an amplified, higher power, and higher amplitude, electrical amplified output signal. Gain associated with power amplifier is defined as the ratio of the output power of the amplified output signal to the input power of the input signal, in dB. In terms of judging power amplifier performance under realistic operating conditions, an Adjacent Channel Power Ratio (ACPR) measurement is used. Prior to taking an ACPR measurement, a desired frequency channel is modulated using a digital modulation scheme, such as that set forth in adherence to the 802.11 b modulation standard. Power amplifier output power in an adjacent channel, with respect to the desired frequency channel, is measured, where the ACPR is a ratio of electrical power in a desired frequency channel compared to that in another adjacent channel, thus giving an indication of frequency spreading of the modulated input channel. If the ACPR is high then spectral regrowth occurs, where electrical power is injected in frequency bands adjacent to the modulated input channel.
Another useful measure of power amplifier performance deals with a measure of a linearity of the power amplifier response. Most amplifiers are used in the linear region, where operating the power amplifier in this region provides low distortion and low harmonics for the amplified signal. In this linear region, the output power is the sum of the input power and the gain. As input power increases, the output power will increase proportionally to the gain until it starts to compress. When the difference between the small signal gain and the actual gain is 1 dB. called the one dB compression point, or P1 dB, operating the power amplifier below this point will cause high distortion and harmonics on the amplified output signal. Therefore, most power amplifier systems are operated a few dB above this P1 dB point
In prior art circuits, power amplifier gain control is provided by one of two techniques, where in the first technique an amplitude of an input signal is fixed while a bias signal used to control a bias point for each amplifying stage is adjusted to provide a desired gain, or in the second technique the bias point for each amplifying stage is fixed with a fixed bias signal and the input signal is varied in amplitude instead of varying the gain. Both of these techniques providing a same amplitude output signal, however, they both have their limitations. The second technique is rarely used at radio frequency (RF), frequencies. In either the first or the second technique one of the input signals applied to the power amplifier is fixed and the other is variable.
Unfortunately both techniques have their disadvantages when used in power amplification purposes. The disadvantage of the first technique is that since the input signal is fixed in amplitude, varying the bias signal past a certain point will distort the output signal because the amplifying stages within the power amplifier are working within a non-linear region, operating close to the P1 dB point. Where in RF applications ACPR will occur as a result when using this technique.
The disadvantage of using the second technique lies in varying the amplitude of the input signal prior to providing the signal to the amplifier gain stages. Where if the amplitude of the input signal falls below a minimum amplitude, or above a maximum amplitude, will result in the power amplifier output signal to also be distorted. The distortion is a result of the amplifier amplifying noise in the input signal when the input signal amplitude is below the minimum amplitude. Where as when the input signal is above the maximum amplitude the power amplifier gain stages respond in the non-linear region. Where the signal is above a maximum amplitude the amplifier operates close to the P1 dB point results in a similar problem of ACPR when used in RF applications.
In some cases, prior art circuits utilize a combination of varying the input signal amplitude and also the gain signal amplitude. Unfortunately, there are typically only a few gain signal amplitudes used for controlling the gain of the gain stages, and as a result when the gain is incrementally varied in amplitude, causing transistors within the gain stage to operate using another gain curve, amplified signal transients will be generated in the amplified output signal as a result of transitions from one gain curve to another during operation. Although this combination provides a form of dual variable gain control, it still compromises spectral linearity and power consumption efficiency for ease of control using a single control signal to vary the gain signal provided to the gain stages. Unfortunately, the aforementioned techniques suffer in that they do not provide maximum power amplifier dynamic range as well as offering decreased electrical power consumption by the power amplifier.
It is therefore an object of the invention to provide a method of adjusting gain and current consumption of a power amplifier such that the power amplifier operates within a linear gain region.
In accordance with the invention there is provided an amplifier comprising:
a first input port for receiving an input signal;
a second input port for receiving a control signal;
a signal power varying circuit for varying a power level. of the input signal in dependence upon the control signal to provide a signal for amplification;
an amplifying stage, having a small signal gain, coupled to the first input port for receiving the signal for amplification and for generating an amplified signal at an output port, the amplified signal being an amplified version of the signal for amplification;
a biasing circuit for receiving the control signal and for biasing the amplifying stage through a bias port using a bias signal, in dependence thereon,
wherein, in use, the signal power varying circuit and the biasing circuit act in cooperation to provide a predetermined power profile for amplified signals and small signal gains.
In accordance with an aspect of the invention there is provided a method of controlling a power amplifier circuit for providing an amplified signal having an output power comprising the steps of:
providing a control signal;
providing an input signal having an input power to an input port of the power amplifier circuit;
adjusting in dependence upon the control signal a bias signal provided to the power amplifier circuit to vary the actual gain thereof;
attenuating the input power of the input signal in dependence upon the control signal to vary the input power of the input signal to the power amplifier to provide a predetermined power profile for amplified signals and small signal gains; and,
amplifying the input signal to form the amplified signal.
In accordance with another aspect of the invention there is provided a method of calibrating a power amplifier circuit, for providing an amplified signal having an output power, comprising the steps of:
providing an input signal having a predetermined input power to an input port of the power amplifier circuit;
providing a mapping circuit;
providing an amplifier circuit, having a small signal gain and an actual gain, disposed within the power amplifier circuit;
measuring the output power of the amplified signal;
adjusting a control signal provided to the amplifier circuit to vary the actual gain thereof until a difference between the output power of the amplified signal and the small signal gain of the amplifier of is 1 dB or less; and,
storing a relationship between control signal data, derived from the adjusted control signal, and bias signal data, derived from the bias signal, within the mapping circuit.